1. Field of the Invention
The present invention relates to a thermally assisted magnetic recording method for implementing heat-assisted information recording on a magnetic recording medium.
2. Description of the Related Art
Magnetic recording media (magnetic disks) are known as recording media for constituting recording devices such as hard disks. The increase in the information processing volume in computer systems creates a growing demand for magnetic disks with increased recording density.
When information is recorded on a magnetic disk, a magnetic head for recording is disposed close to a recording magnetic film of the magnetic disk, and a recording magnetic field with an intensity higher than the coercive force of the recording magnetic film is applied to the recording magnetic film with the magnetic head. When the direction of the recording magnetic field from the magnetic head is successively inverted, while the magnetic head is moved with respect to the magnetic disk, a plurality of magnetic domains with successively inverted magnetization direction are formed in a row in the circumferential direction or in the track extension direction of the magnetic disk. The prescribed signals or information are thus recorded as changes in the magnetization direction in the recording magnetic film.
In the technological field of magnetic disks, it is well known that the higher is the coercive force of the recording magnetic film, the higher is the thermal stability of magnetic domains formed in the recording magnetic film and the easier is the formation of stable magnetic domains of a very small size or width. The smaller are the smallest magnetic domains that can be formed with good stability in the recording magnetic film, the higher is the recording density that can be obtained in the magnetic disk.
In information recording on magnetic disks, as mentioned above, signal magnetic domains cannot be adequately formed unless the recording magnetic field applied to the recording magnetic film is higher than the coercive force. For this reason, the intensity of the recording magnetic field applied by the magnetic head apparently has to be increased with the increase in the coercive force that is set for the recording magnetic film. However, the intensity of the recording magnetic field that can be applied by the magnetic head is limited, for example, from the standpoint of magnetic head structure or power consumption.
Accordingly, a thermally assisted magnetic recording method is sometimes used for information recording on the magnetic disks. When information recording on magnetic disks is implemented with the thermally assisted magnetic recording method, first, the temperature of the recording magnetic film of the magnetic disk is locally and successively raised by conducting laser beam illumination from the prescribed optical head. In the area with the raised temperature in the recording magnetic film, the coercive force decreases with respect to that in the surrounding areas where the temperature has not been increased. Then, a recording magnetic field that is stronger than the coercive force in the area of the recording magnetic field with the increased temperature is applied to the area with increased temperature by the magnetic head, and part of the area with increased temperature is magnetized in the prescribed direction. This magnetization is fixed in the process of cooling the magnetized zone. If the thermally assisted magnetic recording method is used, information recording is implemented by applying the recording magnetic field to the zones in which the coercive force is decreased by heating. Therefore, even when the coercive force of a recording magnetic film at a normal temperature during information storage or information reproduction is set at a high level, it is not necessary to increase excessively the intensity of the recording magnetic field that has to be applied with the magnetic head. Such a thermally assisted magnetic recording method is described, for example, in Japanese Patent Applications Laid-open No. H6-243527 and 2003-157502.
On the other hand, when a plurality of concentric circular tracks are magnetically constituted in the recording surface (recording magnetic film) of a magnetic disk, the user data areas and servo pattern areas are arranged alternately in each track in the extension direction thereof (that is, the circumferential direction of the magnetic disk). The user data areas are the zones where the user data can be rewritably recorded with the magnetic head of the magnetic disk device. The servo pattern areas are the areas for magnetically forming or recording the prescribed servo patterns for positioning and controlling he magnetic head on the target rack on the recording surface. During information recording on a magnetic disk, the user data are recorded in the user data areas, but the servo pattern located in the servo pattern areas is not rewritten.
FIGS. 4A and 4B show an example of the conventional thermally assisted magnetic recording method. FIG. 4A shows a partial cross-section of a magnetic disk 40 rotated during information recording and a slider 50 disposed opposite the magnetic disk. The magnetic disk 40 has a laminated structure comprising a disk substrate 41, a recording magnetic film 42, and a protective layer 43 and is composed as a magnetic recording medium capable of implementing information recording (magnetic recording) and information reproduction in a thermally assisted recording system. The slider 50 comprises a slider body 51, a converging lens 52, and a magnetic head 53 for recording. The slider body 51 comprises a prescribed laser emission section 51a on the side thereof facing the medium. A laser beam L that is emitted from a light source (not shown in the figures) and passed through the converging lens 52 can be emitted from the laser emission section 51a. The converging lens 52 is used to converge the laser beam L. The magnetic head 53 serves to apply the prescribed recording magnetic field Hr to the recording magnetic film 42. The movement direction of the slider 50 with respect to the rotating magnetic disk 40 is shown by arrow D.
FIG. 4B-(a) is a partial enlarged plan view of one track T in the recording magnetic film 42 and vicinity thereof (the scale in this figure is different from that in FIG. 4A). In the track T, the user data areas Y and servo pattern areas S are arranged alternately in the extension direction of the stack. FIG. 4B-(b) is a graph illustrating the control mode of laser illumination in the present conventional method. In the graph shown in FIG. 4B-(b), the position on the recording magnetic film 42 or track T (position in the track extension direction) that is opposite the laser beam emission section 51a located directly below the converging lens 22 is plotted against the abscissa, and the laser power is plotted against the ordinate. The laser power corresponding to the position in the track extension direction of the laser beam L illuminated on the recording magnetic film 42 is represented by a solid line 61. FIG. 4B-(c) is a graph showing the control mode of recording magnetic field application and the changes in the coercive force of the recording magnetic film 42 in the present conventional method. In the graph shown in FIG. 4B-(c), the position on the recording magnetic film 42 or track T that is opposite the magnetic head 53 (position in the track extension direction) is plotted against the abscissa, and the absolute value of the intensity of the recording magnetic field Hr and the coercive force Hc of the recording magnetic film 42 are plotted against the ordinate. The absolute value of the intensity corresponding to the position in the track extension direction is shown with a solid line 62 for the recording magnetic field Hr applied to the recording magnetic film 42 (therefore, the application direction of the recording magnetic field Hr is not represented by the solid line 62). Furthermore, the coercive force Hc of the recording magnetic film 42 at the time the magnetic head 53 passes by is represented by a dash-dot line 63.
In the conventional thermally assisted magnetic recording method shown in FIGS. 4A–4B, the prescribed user data are written by laser beam illumination and recording magnetic field application in the user data areas Y successively facing the slider 50 in a state (that is, in a state in which the magnetic disk 40 is rotated in the direction opposite that of arrow D after the slider 50 has been disposed opposite the recording magnetic film 42) in which the slider 50 is moved in the direction shown by arrow D with respect to the recording magnetic film 42, and the laser beam illumination and recording magnetic field application are stopped at the prescribed timing so that the servo pattern magnetically formed in the servo pattern areas S successively facing the slider 50 does not change.
When the user data are written into the user data areas Y (during information recording), the recording magnetic film 42 is illuminated at the prescribed power, as shown in FIG. 4B-(b), with the laser beam L, and the recording magnetic field Hr is applied in the prescribed direction and at the prescribed intensity to the zone where the coercive force Hc is reduced by heating induced by laser beam illumination in the recording magnetic film 42, as shown in FIG. 4B-(c). As a result, the prescribed signal is recorded as the changes in the magnetization direction in the user data area Y.
Then, as shown in FIG. 4B-(b), the laser beam illumination is stopped to avoid heating of the servo pattern area S located forward in the slider movement direction, when the zone in the track T that faces the laser beam emission section 51a comes to a position P2. Even when no external magnetic field is applied, if the coercive force Hc of the servo pattern areas S is decreased by the increase in temperature, then the servo pattern (not shown in the figure), which has been formed magnetically in the servo pattern areas S, can change or degrade under he effect of thermal fluctuations. Furthermore, at the same time as the laser beam illumination has thus been stopped, as shown in FIG. 4B-(c), the recording magnetic field application is also stopped when the zone in the track T that faces the magnetic head 53 comes to a position P1 located in front of position P2. The distance from the position P1 to the position P2 corresponds to the distance from the magnetic head 53 in the slider 50 to the converging lens 52.
Then, as shown in FIG. 4B-(b), the laser beam illumination is started when the zone in the track T that faces the laser beam illumination section 51a comes to a position P3 of a transition from the servo pattern area S to the user data area Y. Further, as shown in FIG. 4B-(c) the recording magnetic field application with the magnetic head 53 is started when the zone in the track T that faces the magnetic head 53 comes to a position P4 in which the coercive force Hc is sufficiently reduced due to heating by the laser beam illumination. Writing of the user data is thus restarted.
In the track T, the servo pattern areas S are the recording prohibition areas where the recording of user data is prohibited, and the user data areas are the recording areas providing a field for recording the user data. However, with the above-described conventional thermally assisted magnetic recording method, as shown in FIG. 4B-(a), non-recording areas N1, N2 where the user data are not recorded though those areas are essentially contained in the recording areas are formed over a rather large total surface area in the user data areas Y on the boundary with the servo pattern areas S. Such a formation of the non-recording areas N1, N2 on a rather large total surface area is undesirable from the standpoint of increasing the recording capacity of the magnetic disk 40.